Previously known processes for the synthesis of diamond are classified as follows according to the starting material.
(1) Hydrocarbon is used as the raw material
The chemical vapor phase deposition method for decomposing a gas by an energy of heat, electron beam, plasma or the like to form carbon atoms in an activated state and deposit diamond on a substrate.
(2) Graphite is used as the starting material
1. The ion beam method for forming positive ions of carbon from graphite according to the discharge technique, accelerating and focusing the ions and causing the accelerated and focused ions to impinge against the surface of a substrate to deposit diamond thereon.
2. The chemical transportation method for locating graphite, hydrogen and a substrate in a sealed reaction tube, placing graphite at a high-temperature portion and the substrate at a low-temperature portion, applying an energy of heat or the like to the hydrogen gas to generate atomic hydrogen and depositing diamond on the surface of the substrate.
According to the above-mentioned chemical gas phase deposition method (1), a diamond film can be formed at a growing speed of 0.1 to 3 .mu.m per hour on the surface of the heated substrate if the operation is carried out under a reduced pressure (10 to 100 Torr) at a hydrocarbon concentration of 0.1 to 1.0%. However, this method is defective in that the range of the conditions for the synthesis of diamond is narrow, a deposition of graphite or non-diamond carbon often occurs and the diamond deposition speed is low. Especially, in case of hydrocarbons which consist solely of carbon and hydrogen, the kinds of usable starting materials are limited.
The above-mentioned ion beam method (2)-1 is advantageous in that diamond can be deposited on the surface of the substrate at normal temperature, but is defective in that an ion beam-generating apparatus and a focusing apparatus are expensive, the atom of an inert gas used for the discharge is trapped in the diamond film, and i-carbon is deposited simultaneously with diamond.
Since the above-mentioned chemical transportation method is a closed tube method in which a hydrocarbon formed by the reaction between graphite and atomic hydrogen in the closed tube is utilized, a continuous operation is impossible and the thickness of a diamond film that can be synthesized is limited. The method is also defective in that the diamond synthesis conditions such as the concentration of the reaction gases and the heating temperature cannot be independently controlled.
In the process disclosed in Japanese Examined Patent Publications No. 59-27753 and No. 59-27754, in which diamond is synthesized by using hydrogen and a hydrocarbon alone, the kinds of applicable starting gases are extremely limited, and the gases that can be practically used are methane, ethane, and ethylene alone.